The death toll at Chok Chai Furniture, a converted row of four shophouses at the junction of Amphor Road and Srisudhat Road, Talad Yai, included a nine-year-old boy.
Firemen were alerted to the blaze about 2.30am and five truck took two hours to control the blaze, which ripped through all three levels.
With wood to feed on, the blaze quickly spread from the ground floor to the top floor, where a family lived.
The dead are believed to have been asphyxiated.
Killed were three members of a family, Mongklon Rungchai, 43, Nuchari Sangprakon, 43, and nine-year-old Theerawat Rungchai. A cousin, Chatya Kanpat, 59, died with them.
A woman, Janhorm Jakkeaw, 51, jumped from the top floor and was badly injured. She was being treated at Vachira Phuket Hospital.
The adults killed are believed to have been workers at the furniture shop. The owner did not live on the premises.
Police from Phuket City are investigating the cause of the blaze.
It's the worst fire on Phuket since the Tiger Disco blaze claimed the lives of two foreigners and two Thais in Soi Bangla, Patong, in 2012.